The Wellcome Collection sits at the intersection of medicine, art, and the deeply strange, the kind of museum that rewards a whole afternoon and leaves you with a lot to think about on the bus home. The Barbican Conservatory is one of London's most peaceful rooms when you can find it open.
Kelvingrove in Glasgow is free, vast, and full of things you didn't know you needed to see. Every city in this list has a version of this, somewhere to go alone and feel genuinely fine about it.